Wireless local area networks (WLANs) are gaining in popularity, and the demand for WLAN bandwidth is growing. The original WLAN standards, such as IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b, 802.11g and 802.11n extensions to the original standard, in order to enable higher data rates. The 802.11a standard, for example, provides data rates up to 54 Mbps in the 5 GHz band using a multi-tone, orthogonal frequency division multiplexing (OFDM) modulation. The 802.11b standard defines data rates up to 11 Mbps using a single-carrier modulation scheme in the 2.4 GHz band. The newer 802.11g standard permits data rates up to 54 Mbps in the 2.4 GHz band using OFDM, and it is also backward-compatible with legacy 802.11b products. The emerging 802.11n standard allows up to 130 Mbps mandatory support with at least two transmit antennas, and thus it extends WLAN devices to support multiple-input multiple-output (MIMO) technology. Optional modes in this evolving standard allow achieving data rates up to 600 Mbps. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.
In 802.11 WLANs, a fixed access point communicates on a predetermined frequency channel with wireless clients (also referred to as “stations”) in its vicinity. The group of stations communicating with the same access point is referred to as a basic service set (BSS). Generally, with the exception of certain broadcast messages, the access point can transmit downlink signals only to one station at a time. Otherwise, the stations would receive multiple, interfering signals, which they would then be unable to decode. Similarly, to prevent simultaneous uplink transmissions, the 802.11 standard provides a contention avoidance mechanism, which requires a station to refrain from transmitting when it senses other transmissions on its frequency channel. However, when operating in license-exempt frequency bands, there may be other devices operating on the same band as the access point, which may not follow the contention avoidance mechanism, and thus create interference to the access point (AP) and clients.
The characteristics of the interference existing in license-exempt bands such as the 2.4 GHz may vary in time and nature. The interference may have fast time variation, with a coherence time close to one packet duration, or it may have a slow time variation, with a coherence time of multiple packets. The bandwidth of the interference, measured with respect to the WLAN signal bandwidth, may also vary, with both wideband and narrowband interference being common. This means that the interference environment of a WLAN AP is unpredictable and can have widely varying characteristics.